Josh Brolin does a pretty damn good Tommy Lee Jones in "Men in Black 3." Good thing, because the actual Tommy Lee Jones is barely in the movie, and when he is, it seems he’d rather not be.

By now, Jones can do the stoic, perpetually unfazed Agent K in his sleep, and that’s more or less what he does. Brolin is a different story. Playing a younger Agent K — in July 1969, where Agent J (Will Smith) has time-jumped to prevent an alien marauder from killing K — Brolin not only brings some Jonesian dry wit to the role but suggests a fresher, more optimistic K. He alone makes MIB3 a worthier sit than the previous sequel.

Beyond that, there’s Smith doing his usual shtick as J, who you’d think would be used to extraterrestrial shenanigans after fifteen years, but who reacts to everything the same way he did in 2002 and 1997. J has somehow kept his humanity in his job, but how? How do you deal with surreal threats to Earth every day for a decade and a half and not turn into a jaded cold cod like K? The movie isn’t interested in that; it’s more concerned with its Moebius-shaped timeline, in which the alien villain (Jemaine Clement) seeks to kill K before K can implement a shield to keep the villain’s cohorts from invading Earth. J isn’t even supposed to interact with the younger K, but he’s forced to, and the movie doesn’t get into any possible catastrophic consequences that may result from J being in 1969 — or any benefits, either.

There’s an intriguing character named Griffin (Michael Stuhlbarg), who sees all possible futures. I almost wanted MIB3 to break off and become a mockumentary about him — his butterfly-effect way of seeing life as infinite branches that can point to glory or doom based on whether someone leaves a tip at a diner. That’s the problem with the MIB movies — they serve up fascinating concepts, but they all take a back seat to the same chase scenes, the same shoot-outs with space-age weapons. Make-up wizard Rick Baker reportedly built a bunch of retro-looking aliens for the 1969 scenes, not that we get to see much of them. Mostly we’re stuck with Jemaine Clement’s Boris the Animal, who growls and shoots people with spines launched from his palms. There are two of him, too — the one from 2012 and the one in 1969 — so he gets tiresome fast.

MIB3 reportedly cost $250 million, though it doesn’t look much more expensive than the earlier films. The 3D, as usual ladled onto the film after shooting ended, doesn’t help. At this point, I think I’d rather skip such post-converted 3D movies — generally you miss nothing by opting for the 2D screenings — and hold out for the ones designed for 3D and actually filmed in 3D, like the upcoming Prometheus. The script, by Etan Cohen and the uncredited David Koepp and Jeff Nathanson, tries for some emotional depth with the fatherless J looking fruitlessly to K for some caring and sharing, but once J gets together with the younger K that aspect gets lost, only to be rediscovered in a last-act twist.

By that point we want to see J and the older K reunited, and we do, briefly, but there’s no weight to it. I suppose the point — and, for some, the appeal — of these movies is that they’re weightless romps. In theory, and with the cast of eccentrics the MIB series have attracted with a big paycheck, the movies should be nutball classics. But most often what they boil down to is Will Smith getting flung around by some giant beast, or Tommy Lee Jones smacking someone repeatedly with an alien fish.

If that’s what hits your funny bone, bon appetit. The hip, knowing backdrop of the films — their winking acknowledgment that what you suspect about aliens is true — is more interesting than the run-of-the-mill plots scampering around in front of it.